The present invention relates to a device for receiving a closed caption broadcasting signal for display on a television (TV) screen. Closed caption broadcasting is provided for those viewers in the United States who have hearing disabilities.
A closed caption broadcasting system can display a closed caption containing dialogue, announcements and the like on a TV signal being displayed on a TV screen. The U.S. closed caption system, which is quite similar to the Japanese system, is a multiple character broadcasting system. In this system, closed caption data is transmitted and superimposed within a vertical blanking period of a television signal. The data transmission of the closed caption data is based on a code system.
A character generator in a TV receiver responds to codes in the transmitted signal to display the desired characters on the TV screen the moment the corresponding codes are transmitted.
There are generally two kinds of display modes available for displaying closed caption data, Text Mode and Caption Mode. In Text Mode, a maximum of 15 lines by 32 characters can be displayed in the center of the TV screen. In Caption Mode, the closed caption characters can be displayed only on upper or lower portions of the TV screen, for example, in a space between the 1st and 4th TV scan lines or a space between the 12th and 15th TV scan lines.
Referring to FIG. 2(a), the closed caption broadcasting data are superimposed in a vertical blanking period 21 H (H means a horizontal scanning period. 21 H means the 21st horizontal scanning period during the vertical blanking period i.e. line 21) of the transmitted TV signal being displayed. Vertical blanking period 21 H is shown as a composite signal containing a 3.58 MHz color burst signal, a 503 Khz run-in clock and a predetermined amount of closed caption data.
Referring to FIG. 2(b), the closed caption data includes a 3-bit start signal followed by an 8-bit string consisting of a data bit string of 7 bits plus a parity bit. The 8 bits of data, including the parity bit, may be considered as one unit which commands the character codes and a command code defining a position (row and column) or color of the characters to be displayed.
Referring now to FIG. 3 a conventional receiver for displaying the closed caption data in the closed caption broadcasting system includes an antenna 101 which applies the television signal to a tuner 102. An intermediate frequency amplifying circuit 103 amplifies and detects the television signal to produce a detected output for connection to a Y/C (Luminance/Chroma) processing circuit 104 which applies the resulting primary colors of Red(R), Green(G) and Blue(B) to an RGB switching circuit 105.
The detected signal from intermediate frequency amplifying circuit 103 is also applied to a sampling circuit 106 which passes only the caption data in 21 H to a decoder 107. Decoder 107 conducts a parity check on the caption data, to confirm that it receives valid closed caption data from the television signal. When the parity check is successful, decoder 107 applies the decoded caption data to a character generator 108. Character generator 108 produces the desired closed caption characters for display at a desired position on the screen.
The output of character generator 108 is applied to an output control circuit 109. Output control circuit 109 applies the three primary color signals RGB necessary to display the characters, and to control their color, to switching circuit 105. The desired closed caption characters are thus displayed on the TV screen.
Sampling circuit 106 samples the caption data under control of a clock actuated counter. The clock is synchronized with the television signal for correct display of the closed caption data on the TV screen. To assure synchronization of the clock, it is convenient to use an Automatic Frequency Control (AFC), such as shown as circuit 3 (of FIG. 1), phase locked to the horizontal synchronizing signal in the transmitted television signal. More specifically, a signal as shown in FIG. 2(c), derived from a frequency dividing circuit in automatic frequency control circuit 3, is compared with the horizontal synchronizing signal in FIG. 2(a). Based on the results of the comparison, the frequency of the clock signal is controlled to equal the horizontal frequency of tile television signal.
Accurate synchronization of the automatic frequency control circuit with the run-in clock is vital for proper display of caption data. Incorrect synchronization may prevent correct decoding of the caption data, thereby preventing accurate sampling and display of the caption data.